Process for producing dispersion strengthened alloys



United States Patent US. Cl. 75-206 4 Claims ABSTRACT OF THE DISCLOSURE A method of dispersing oxide particles in a metal matrix to form dispersion strengthened alloys. A sol is prepared from the oxide particles and mixed with a gel to suspend the oxide particles and powdered metal is added to the resulting suspension. The suspension is then dried and sintered to form the alloy.

The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention concerns a new and useful process for effecting a uniform distribution of submicron oxide particles in a metal matrix by means of stable suspensions to produce dispersion strengthened alloys.

The main object of this invention is to determine how colloid chemistry may be applied in obtaining a uniform dispersion of submicron thoria particles in a nickel metal matrix. Of the metal-oxide systems made and hot rolled were those of the compositions Nl-I-ThOg, Ni-l-Al O Ni-i-Baymal, Cu-l-Tho Cu+Al O and Cu+Baymal. All systems were investigated by a systematic processing schedule of: gel preparation, drying, purification, cold pressing, sintering and hot-rolling.

The tremendous strides in technological progress has always presented an ever increasing demand for materials having increasingly favorable strength-to-weight ratio; improved corrosion resistance in hostile environments; and an increasingly broad spectrum of physical properties. The jet engine, ballistic missiles, space vehicles and atomic power plant developments impose most stringent demands on the materials of both their structure and their use.

Titanium melts at 1800 C. and vaporizes at temperatures above 3000 C. Manned aerospace vehicle wing leading edges entering the atmosphere considerably exceed temperatures of 1093 C. or 2000 F. Vanadium steels have melting points in the order of 1475 C.

Refractory metals, ceramics and the like, are used successfully at 2000 F. Very few nickel and cobalt base alloys exhibit useable strength above 1800 F., although some that are useable up to 1900 F. are limited to cast forms.

An approximation of 20 pounds of simple launching vehicle on the ground, for every pound of payload put in orbit, compares with a ratio of 100 to 1 for manned space vehicles with control surfaces for pinpoint landing. Metals and alloys with the lowest possible densities commensurate with a need are favored for high temperature service.

The present invention comprises the use of a gelatinous mass produced by combining a stable sol combined with a gel developed with lubrication techniques for high temperature extrusion. The particles used are dimensioned at less than 0.05 micron in diameter. The particles comprise a substantially uniform distribution of chosen stable metal oxide particles throughout a metal matrix.

Illustrative mixes comprise Cu-I-ThO Cu+Al O Ni+ThO Ni+Al O Cu-i-Baymal and Ni+Bayma.l. The term Baymal is a Du Pont tradenarne for the material that consists by weight of 83% AlOOH, 9.8% CH COOH, 5% H 0, and 1.7% $0,. The mixing of the charge of metal particles with oxide particles is sensitive to the pH of the solutions and the gels that are used and to the method of evaporating and drying the product. The process is applicable to a wide range of inert particles of which oxides, borides, and nitrides are illustrative.

The hardening and the strengthening of metals is believed to be materially assisted by arresting slip, according to the theory of line defects or dislocations, with a particle size of 10 atomic diameters constituting a critical dispersion of hard particles in a relatively soft matrix.

The optimum strength-to-weight ratio for any metal base system at temperatures approaching of the material melting point are attained in dispersion strengthened alloys by the addition of an oxide or other inert phase.

The present invention is directed toward the uniform dispersion of an insoluble phase into a metal matrix. Internal oxidation techniques are considered to be limited to material with a small cross section and dependent on the diffusion of oxygen through the specimen. Current co-precipitation and preferential reduction processes are limited by extremely sensitive pH control and chemistry complexity.

Only a relatively few experimental. results have been presented, however they are considered to be the most illustrative. Many early investigations are seriously hindered by the lack of metallic powders with particle sizes less than 40 microns. Consequently, the small interparticle spacing and extremely fine grain size derived in SAP and TD. nickel were never achieved. Even with the best dispersion strengthened alloys produced, extremely sophisticated experimental tools are required to probe for an understanding of the mechanisms of creep, recovery, recrystallization and fracture. A great deal of elfort is required before an adequate understanding is achieved.

Dispersion strengthened alloys exhibit superior high temperature strength capabilities. The mechanical properties of the end product are sensitive to the consolidation and the conversion parameters used. The interparticle spacing should be less than 1 micron. The dispersed phase should have negligible solubility, a high hardness, and a high free energy of formation. Strengthening increases almost linearly with the amount of dispersed phase added and a decrease in ductility also follows almost proportionally. For an optimum compromise of strength and ductility, approximately from 2 to 6 volume percent of the dispersed phase is necessary in the alloys.

The fundamental principles of colloid chemistry that promote stability in dilute solutions is supplied to some advantageous degree to effect more uniform dispersion between oxide particles and metallic powders.

A stable thoria sol was prepared from a thoria gel by a. repetitive sol-gel process. The gel dissolves readily in water and the particle size of the thoria in suspension is approximately 50 A. in diameter. The colloidal boehmite (AlOOH) alumina or Baymal has the chemical composition by weight of AlOOH 83%, CH COOH 9.8%, H 0 5% and S0 1.7%. Its morphology is described as being clusters of minute fibrils with a pore diameter of 77 A.

In the preparation of gels, 1% by weight of gelatin was added to boiling water. The boiling condition was maintained for the first several minutes of stirring to in sure complete dispersion. Metal powder was added while the gel was in a hot fluid state. Stirring was continued after the heat was removed and until a relatively high viscosity was observed. Total stirring time was approximately one hour.

A thoria suspension was made by adding warm distilled water to thoria gel, bringing the resulting dispersed thoria sol to a boil and immediately pouring it over dry agar.

r 3 The pH of the thoria sol and the agar gel was maintained between 2.5 and 3. Careful adherence to the pH 2.5 to 3.0 range was maintained throughout subsequent mixing.

No precipitation effects were ever noted when adding metallic powders to the hot agar-thoria gel. The powders were added slowly in approximately one gram batches. Additions of one micron nickel and copper powders were very easily accomplished.

EXAMPLE A typical batch preparation of one micron nickel powder plus 3.4 percent by volume of thoria was accomplished by placing 1.64 grams of thoria into 125 ml. of warm water. Stir intermittently and heat on a hot plate below boiling until it is evaporated to 75 ml. of thoria sol. Adjust the thoria sol with dilute nitric acid of a pH of 2.5 to 3.0.

Adjust 100 ml. of water to pH 2.5 to 3.0 with dilute nitric acid. Bring the water to a boil and pour it over one gram of dry agar in a 250 ml. beaker. Start stirring the agar water solution immediately and keep it at the boiling point until the agar is dispersed completely as a gel. Reduce the heat to just below boiling and maintain constant stirring of the agar gel.

Transfer part of the 75 m1. of thoria sol to a 50 ml. burette. Adjust the stopcock on the burette to permit rapiddrop flow into the agar gel. Continue the operation until all the thoria sol is dispersed. Stir the resulting sol for five minutes for adequate mixing.

Add metal powder in approximately one gram batches while the temperature of the thoria sol is maintained at just below boiling until 41.27 grams of nickel powder are added to the sol. Keep stirring the suspension for fifteen minutes for a homogeneous gel.

Remove the heat, but keep stirring for thirty minutes until the sol assumes a high viscosity.

Scrape out the gel on a hot nickel plate. Maintain the plate temperature at from 80 to 90 C. Permit the nickelthoria sol so made to dry for a minimum of 8 hours before removing it from the plate.

All powders used were purified prior to their being used. The nickel powders were purified at a temperature not exceeding 450 C.

SAP is an abbrevation for sintered aluminum powder ground to a fine flake of approximately 0.01 to .2 micron. T.D. nickel is made by the coprecipitation of nickel hydroxide and thorium oxalate reduced ina hydrogen atmosphere producing powders of metallic nickel and thoria. Sintering of copper and the oxides cited is accomplished in a hydrogen atmosphere at 950 C. for a minimum of two hours.

It is to be understood that the process and the products that are disclosed herein have been submitted as being illustrative successful reductions to practice of this invention and that limited modifications may be made therein without departing from the spirit and the scope of the present invention.

I claim:

1. The process for uniformly distributing submicron thoria particles in a nickel matrix and forming a composite; said process comprising the steps of:

(a) preparing a gel by adding one percent by weight of agar to boiling water, said boiling water having its acidity adjusted to a pH of 2.5 to 3 by the addition of nitric acid;

(b) preparing a thoria sol by adding thoria to warm water in the ratio of about 1.6 grams of thoria to about 125 grams of warm water, boiling away about of said water and adjusting the pH of the resulting sol to 2.5 to 3 by adding dilute nitric acid;

(0) forming a mixture from said thoria sol and said gel by adding 50 parts by volume of said thoria sol to 100 parts by volume of said gel while said gel is in a hot fluid state and stirring to thoroughly mix;

((1) forminga suspension of nickel powder in said mixture by adding nickel powder in approximately one gram batches to said mixture while said mixture is held at a temperature just below boiling until about 41.2 grams of nickel have been added for every 1.6 grams of thoria;

(e) allowing said suspension to cool While stirring;

(f) drying said suspension; and

(g) sintering said suspension to form a composite.

2. The process of claim 1 wherein nickel is replaced by copper.

3. The process of claim 2 wherein thoria is replaced by alumina.

4. The process of claim 3 wherein copper is replaced by nickel.

References Cited UNITED STATES PATENTS 3,143,789 8/1964 Iler et a1 29-1825 3,180,727 4/ 1965 Alexander et al. 20'6 X 3,218,135 11/1965 Alexander et al. 75--206 X 3,317,285 5/1967 Alexander et al 29182.5 3,326,677 6/1967 Alexander et al. 75-206 3,386,814 6/1968 Alexander et al. 75-206 X CARL D. QUARFORTH, Primary Examiner M. I. SCOLNICK, Assistant Examiner U.S. c1. X.R. 

